When wearing a head-mounted display (HMD) in everyday environments, interactions with real world bystanders often fail due to the visual barrier. As a result, the HMD user takes off the headset, which ends the virtual reality (VR) experience. We address this problem by providing a shared surface with the same content for both users, which is located at the same physical position in the real and the virtual world. In a between-subject user study ($N=40$), we investigate the effects of a shared surface for short-term collaboration in co-located mixed-presence scenarios. We compare (a) real-world collaboration, (b) having a shared surface only and (c) combining the shared surface with an avatar representation of the real world user in VR. We could show that shared surfaces are helpful for mixed-presence collaboration. Adding an avatar in VR improves performance measures such as task-completion time, error rate and number of clarifying questions. To support future work in this field, we finally propose design implications and research directions.